fix: handle basepath rewriting for socket connection

kibana/elastic#16724 added a basepath rewrite to the kibana server, and any route that doesn't go through hapi needs to deal with it explicitly

__tests__/fixtures/kibana.js

@@ -1,4 +1,4 @@
-import get from 'lodash';
+import { get, has } from 'lodash';
 import mockElasticsearch from './elasticsearch_plugin';
 
 const config = {
@@ -18,9 +18,10 @@ export class Plugin {
         elasticsearch: mockElasticsearch,
       },
       config: () => ({
-        get: (key) => get(config, key),
+        get: key => get(config, key),
+        has: key => has(config, key),
       }),
-      route: (def) => this.routes.push(def),
+      route: def => this.routes.push(def),
     };
 
     const { init } = this.props;

server/routes/socket.js

@@ -6,7 +6,13 @@ import { typesRegistry } from '../../common/lib/types_registry';
 import { getAuthHeader } from './get_auth/get_auth_header';
 
 export function socketApi(server) {
-  const io = socket(server.listener);
+  const config = server.config();
+  const socketPathPrefix =
+    config.has('server.rewriteBasePath') && config.get('server.rewriteBasePath')
+      ? config.get('server.basePath')
+      : '';
+  const socketPath = `${socketPathPrefix}/socket.io`;
+  const io = socket(server.listener, { path: socketPath });
 
   io.on('connection', socket => {
     console.log('User connected, attaching handlers');